Air sound apparatus



March 1, 1932.

H.HECHTETAL AIR SOUND APPARATUS Filed April 17, 1928 2 Sheets-Sheet 1 ENERGY IN WHTT FREQUENCIES March 1, 1932. H. HECHT ET AL 1,847,873

AIR SOUND APPARATUS Filed April 17, 1928 2 Sheets-Sheet 2 no I iatentecl Mar. ii, 11932 amt Em'BICH. HEGHT AND ULRICH JOHN, OF KIEL, GERMANY, ASSIGNOBS TO ELECTRO- ACUSTIC GESELLSCBLAIT m'r IBESCHRANK'IER HAIFTUNG, OF KIEII, GY, A

am SOUND APPARATUS Application filed April 17, 1928, Serial No. 270,733, and in Germany May 7, 1927.

Hitherto, when it was a matter of solving .e problem of communicating the energy of tuned vibrating diaphragm to the air, at )& efiiciency of transmission, a. resonator [8 been employed as a rule, which is coned with thevibrating diaphragm by makg the latter one wall of this resonator. The sonator is thus, as is already apparent from .e meaning of the above words, tuned to the me frequency as the diaphragm. In order transfer the oscillations, which are pro- 1ced in the resonator by the diaphragm, to Le air, it is necessary to select the form of .e Helmholz resonator, which furthermore |.S rigid walls and a transmission holexin .ese walls. With this arrangement a relatively close lation of the whole apparatus to the temsrature of the air in the resonator exists, ace the tuning of the resonator itself fluc- .ates with the temperature of the air, and

1 account of the utilization of the resonator.

ning for the total output of the ap ratus variation in tuning occurs therein so with .e variation of temperature. A. further dif- :ulty in principle of this repeatedl deribed type of sound transmitter resi es in e feature that, on one hand, for a predermined most favourable degree of coupling the resonator with the exciting diaphragm, predetermined resonator volume is preribed, than-however, onthe other hand, for e transmissionof the resonator energy to e air a predetermined best size of hole exists. 11% also the size of the resonator hole is :termined by a particular resonator volume it-h given tuning, the designer does not have 'Lth such a combination of elements, a free md either in the selection of the size of the rle or in the selection of the coupling bereen resonator and diaphragm. It is, therere, in principle impossible in this way to 'oduce the best output results of the device. The invention avoids these drawbacks by ranging infront of the exciting diaphragm special air cushioned plate or transmitting aphragm, which is excited into oscillations the exciting diaphragm through the intersed air layer. The reasons for this second aphragm will be given later. For transknown that directional emission of sound can be obtained by using a plurality of sound radiators arranged in a predetermined position to each other, so that interference phenomena can be made use of. The directional emission of sound then is a consequence of the superposition of sound waves in the open medium. With such an arrangement the effect of the several sound radiators is in some directions added, in others neutralized, so that no sound is radiated in the latter case. We have found that this problem can be solved in a simple way if one and the same mechanical oscillatory structure serves for exciting the difl'erent radiators of such a directional group.

If, for instance, it is desired to radiate sound substantially in one plane and in all directions in this plane with equal power this problem can be solved theoretically in simple and well known manner by arranging similar sources of sound oscillating in phase, in a straight line at right angles to the plane of radiation, the radiators being spaced apart symmetrically above and below the plane of radiation at the distance from one another of half a wave-length of the radiated sound frequency. With such spacing the negative half of the sound ampli-' tude of one radiator arrives at the other radiator when the latter radlates its positive half of the amplitude, because the negative wave, or in case of air the low pressure half of the first radiator must traverse the distance of one half wave length before it arrives at the second radiator. The result of such an arrangement is that in the direction of the aforementioned straight line, the two Mid sound sources are superposed at opposite phase and thus neutralize each other, whereas in the plane at right angles to this direction the phases are unidirectional and the efiect of the two radiators is additive.

The practical carrying out of this wellknown idea is associated, however, with difficulties, which are mainly due to the fact that on account of coupling of the transmitters with one another, it is not possible to work with a plurality of transmitters entirely independent of one another and, furthermore, it is not possible to design the actual radiators in the ideal form of points.

length, the reason for which will be explained stance an antenna oras at present-an later, and that, finally, the oscillator is arranged in the connecting axis of the planes of the' annular radiators.

The relations of the above lengths to the wave-length are subject to certain small corrections, due to the shape of the conduits, so that the lengths in wave-length units are to be regarded as effective lengths. Inasmuch as in the difierent sound chambers of the radiator the sound is timely distributed with unequal amplitude this characteristic feature has been termed as non-quasistationary, an expression also well known in the radio art and designating an oscillatory sys-.-

tem whose dimensions (in the present case the total length of the pipe) are not small compared with the Wave length. For inacoustic pipe of the length is already non-quasistationary. Therefore, in the annexed claims this term will be used to define this particular character of sound chamber.

In the design of such transmitters horntype radiating structures are often used in which the sound has to pass breaks in its ance. Particularly it is never permissible at breaks or sharp bends, in the path of the sound from the mechanical oscillator, such as a diaphragm, to the free medium, to have at such places a reduction in cross-section. In the same way too great an increase in crosssection should never occur, since that would be equivalent to a passage into the free medium, which would take away from the sound conduit its inherent property. It has been found that the best conditions for the relation of successive cross-sections at breaks or sharp bends lie between 1:1 and 1:5 and,

depending upon the angular direction of the sound at the break, small angles being associated with small upward steps in crosssection (at 0 1:1) and large angles being associated with large steps in cross-section (at 18Q 1:5). This law is generally applicable and independent of the actual form of the horn. In the case of branches the cross-sections of the branches are to be added and treated as one cross-section.

In Figures 1 to 6, constructional forms of the invention and their manner of operation are represented.

Fig. 1 shows a simple apparatus according to the invention,

Fig. 2 a resonance curve of such an apparatus,

Fig. 3 a section through a circular radiator with lateral arrangement of the oscillation generator,

Fig. 4 a section through a circular radiator with arrangement of the oscillation generator-above the radiator itself,

Fig. 5 a section along ef of Fig. 4, and

Fig. 6 shows he passage areas of the sound at the breaks I, II, III, IV. of the modification Fig. 4.

In Fig. 1, 1 is a transmitter casing, preferably of circular form, which is closed on one side by the tuned diaphragm 2 determining the pitch of the transmitted tone. It contains the field magnets 3, the coil 4 of which carries the exciting current. The diaphragm appropriately carries a special armature 5 opposite the field magnet which is moved by the latter and sets the diaphram into oscillations. A small distance away, opposite the diaphragm 2, is disposed the intermediate plate or transmitting diaphragm 6 whicl is actuated by the diaphragm 2 through th interposed air cushion. This auxiliary dia phragm is advisable and required in mos1 cases forthe following reasons. The energy transfer from an oscillating body to air i: the greater the smaller the difierence in mas: between the surrounding air and the body For this reason for instance paper dia phragms are so effective for radio loud speak ers. In the present case, first of all, larg amounts of electric energy are to be trans mitted (of'the order of to 1 kw.). Sine this is to be effected electromagnetically, con

' siderable masses of iron are required for the field and the oscillating armature of the oscillation generator. The armature must be fixed to the tuned oscillatory system (diaphragm 2), which, in order to carry such a heavy mass in its center and to be tuned to fairly high audio frequencies at the same time, must be very much thicker and heavier than a similarly tuned diaphragm without such a mass load, so that for the above reasons it is very unsuited for sound transmission to the air. To overcome this difficulty the heavy diaphragm 2 in the present case is used largely as the carrier for the armature and excites through the small air chamber a second, very much lighter armature 6, which is capable of transferring a very much larger amount of oscillating energy to the air. This intermediate diaphragm acts in Fig. 1 upon the open pipe 7, which, when constructed straight, has a length of of the tone to be transmitted, namely, the natural period of the diaphragm 2. A certain improvement in the transmission to the free air at the upper end of the pipe is obtained in addition, if the pipe is given a conical shape such that its sides extend substantially in accordance with an exponential law. By this means a gradual broadening out of the oscillation from the pipe to the surrounding air is obtained.

In the fixing of the width of the lower pipe opening. there is freedom of course within very wide limits, completely in contradistinction to the principle of the Helmholz resonator. In fixing the dimensions of the intermediate plate 6, it should further be stated that appropriately it is made thicker at the edge and thinner in the middle in such a way that the air chamber between it and the main diaphragm is conical. By this expedient an increased vibration of the central part of the intermediate plate 6 acting on the pipe is obtained; moreover, owing to this arrangement this central zone can be,made still lighter than in the case of a plane intermediate plate, and finally the electricity of the interposed layer of air is improved. The thickness of the intermediate plate is appro priatelv so selected, that its natural frequency lies below the frequency to be transmitted If, for the excitation of the main diaphragm, the principle of electromagnetic excitation is employed, as represented in Fig. 1, a condenser C is preferably connected in parallel or in series with the alternating current machine WV for partial compensation of the phase displacement between current and volt-- age, which occurs through the self-induction of the exciting coil 4. The condenser need not be of such dimensions that resonance takes place between the electric supply circuit and the mechanical system, because at the desired operating frequency current or voltage resonance effects may occur during normal operation which in that case may also be equal or almost equal to the natural frequency of the electric circuit, whereby under certain conditions dangers of an electrical character may occur. Moreover, it is preferred to avoid the effect of the natural damping of the electric circuit on the total damping of the mechanical system. Appropriately the value of the condenser is so selected that the natural tuning of the electric circuit is located above the resonance curve of the mechanical sys tem. The resonance curve of such an apparatus then appears as represented in Fig. 2 in which the energy absorbed by the system is plotted in watts against the several frequencies. The peaks n n belong to the coupled mechanical system formed by the main diaphragm and the open pipe. The tuning of the electric circuit is located at N. The exciting frequency of the machine is located in the centre between a, and n Thus a relatively great stability of the machine frequency is obtained, because through the rising of this watt resonance curve to both sides of the operating frequency, the machine is naturally prevented from running away and this advantage is obtained without employing electric tuning. The location of the electric tuning above the tunings of the mechanical system secures the additional advantage that in case the machine should still once run beyond n the loading by the electric circuit absolutely prevents the machine going beyond N. These advantages are secured in no way too expensively through the disadvantage that on account of the detuning of the electric circuit the cos. (p in the supply circuit cannot be made quite equal to unity.

Whilst the sound transmitter of Fig. 1 is one of the non-directional type, the following examples show transmitters of the directional type.

In Figs. 3 and 4, 1 indicates again the casing, 2 the diaphragm, 3 the field magnet, 4 the field coils and 5 the armature of a normal electro-magnetic transmitter on the telephone principle.

The pipe 7 forks in the example of Fig. 3 at the point p into two branches 20 and 21, which extend vertically in the example represented. The two branches terminate in flares 8 and 9 of circular form each of which is provided with a cover 10 and 11, respectively, also of circular form and spaced from the flares. There are thus obtained between the members 8 and 10 and 9 and 11 circular channels,22 and 23, which abut each with a short cylindrical surface against the propagating medium. Of course suitable ribs 22 are provided between each cm'er 10 and 11 and its pertaining flare to hold the covers in position. This cylind rical surface is the actual radiator, which de- 'livers the energy to the surrounding medium.

In the example of Fig. 4 the plpe 7 also forks at the point p into two branches 20 and 21, which in a similar manner as the example of Fig. 3 terminate at 22 and 23 in circular channels. The difference with respect to Fig. 3 is that in this case pipe 7 and branches 20, 21 are arranged concentric to one another. In this manner it is possible to take the oscil: lation generator, which in the example of Fig. 3 partly stands in the field of radiation and thus causes a certain distortion of the horizontal characteristic, out of this field.

The diameter of the circle formed by the radiating cylindrical surfaces 22, 23 must be selected as small as possible. In no case may it be much greater than 1/4 of the wavelength of the operating frequency of the apparatus. The reason for this limitation is as follows On one hand the area through which the sound emanates into the free medium should be as large as possible, because the amount of radiated sound energy depends upon the size of this area. Onthe other hand, in order to attain a superposition of the two opposite wave halves of the two radiators as exact as possible the axial length or the height of each cylindrical area should be as small as possible so that the exact distance between the radiators approaches as near as possible the ideal point to point distance (for instance distance H in Fig. 4). To still attain a large radiating area with small height, one might attempt to increase the circumference of the annular area. There, however, another limitation is encountered. Assume such an annular slot as a radiator in free space and consider two of its diametrically opposite surface elements as radiating elements. It will at once be seen that the sound impulses emanating from these elements in different directions must sometime meet and will superpose upon each other and produce a resulting impulse, depen'ding in phase upon their travelling distance and time. If the diameter of the ring is small for a given wave length, the phase difference of the sound impulses emanating from the elements at the ends of this diameter is very small and the impulses are added. If the diameter is increased and attains one half of the wave length, the phase difference increases until at one half wave length, it is 180. In other words in the latter case the two emanated impulses neutralize each other. This is true for all surface elements of the ring at the ends of an infinite number of diameters. In other words, an annular radiating surface of one half'wave length diameter will virtually not radiate audibly. For this reason the diameter should be for practical purposes not much greater than 4 wave length of the operating freqnency. The lower limit of the diameter is of course given by the necessary passage cross-section of the acoustic pipe 7 and its branches, which again depend .upon the amount of energy to be radiated. The distance of the planes of the cylindrical radiators 22, 23 from one another, measured between the center lines of the openings, must,

as theory requires, be equal to one half wave-.

radiator'as an acoustic pipe is still present,

but then the drawback arises that the apparatus, on account of the detuning of the volume of air in the pipe with temperaturevariation becomes sensitive to temperature.

In 5 a section along the plane e-f through the apparatus according to Fig.4 is represented in order to show how the acoustic pipes 7 and 20, 21 are mechanically brought into relation with one another. This is effected with the aid of the webs 12, 13.

The device according to Figs. 4 and 5 gives a completely uniform horizontal circular field of sound, which radiates upward anddownward, in the position of the radiator represented, only a small amount of energy and in the horizontal plane a very large amount of energy.

In Fig. 6 the cross-sectional forms of the passages at different points of the pipes aredrawn hatched. vCross-section I is an area surface. cross-section II has the form of a cylindrical area, cross-section III at the branching is composed of two annular passages, are leading upward, the other downward, whilst finally cross-section IV has two cylindrical surfaces, one being at 22, the other at 23. The size of these areas is determined by the radius r of the first cross-section, which againis given by the amount of energy to be radiated by the transmitter.

In the present example for the change from I to II an area ratio of 1 2 is assumed,-

since we have in this case a break in direc-. tion of We then derive the height h of the cross-section II, and h of cross-section IV, and the radius R of the large circle of cross-section III (neglecting the wall thickness of the constructional parts of the radiator) as functions of r from the following equations, assuming that the height H which the two cross-sections IV are apart is measured from center to center as shown in Fig. 4, and is equal to one half wave length, as suming. further that the wall thickness of the inner tube 7 can be neglected as well as the flare in which the cross-sections IV end, so that forthe latter we assume'a radius equal to the outer radius R of cross-section III.

We thus derive a value for cross-section I=1' 1r (1'=1adius of I) II 21- 1:- III 41 IV 81% The height h of the cylindrical cross-section II is derived from the equation:

The large radius of cross-section rived as follows: i

R 'zr 1 1r 47 w R 1r 57 1r R 5r R 13/5 The height h of each cross-section IV is obtained as follows, assuming as previously stated that R of IV is equal R of III:

III is de- For the length of the acoustic pipe for the 5 non-directional type, the rule is generally applicable that it need not merely be but it generally may be an uneven multiple of A The excitation of the main diaphragm of the apparatus can, of course, be efiected in any desired manner, for example, mechanically say by blowing on it or electrodynamically, in which case then, of course, the magnet armature must be replaced by a current conductor moved in the alternating field of an exciting coil. The apparatus can also be driven accordin to the condenser princi 1e, particularly or very high frequencies, w erebfy the main diaphragm must form the plate 0 a condenser excited with alternating current or must be connected with such a plate. The manner of excitation is immaterial for carrying out the invention, as is also the manner in which the exciting alternating current is generated.

We claim l. A directional air sound apparatus com prising in combination a vibration generator, an oscillatory structure adapted to be excited by the said vibration generator and being tuned to a definite frequency, and two sound radiators coupled with said oscillatory structure and having their sound paths partly in common, the length of said paths being equal to an uneven multiple of one quarter of the wave lengthof the said frequency, the open ends of the said sound radiators having an annular form located in two parallel planes spaced apart the distance of one half the wave length of the said frequency.

2. A directional air sound apparatus comprising in combination a vibration generator, an oscillatory structure adapted to be excited by the said. vibration generator and being tuned to a definite frequency, and two sound radiators coupled with said oscillatory structure and having their sound paths partly in common, the length of said paths being equal to an uneven multiple of one quarter of the wave length of the said frequency, the open ends of the said sound radiators having an annular form located in two parallel planes spaced apart thevdistance of one half the wave length of the said frequency, the diameter 0 said annular radiator not materially exceeding one quarter of said wave length.

3. A directional air sound apparatus, comprising in combination a vibration generator, an oscillatory structure actuated b said generator and being tuned to a de nite wave length, a sound resonator of the organ pipe type having a length of one half of the wave length of said frequency and being coupled with said oscillatory structure, a second pipe resonator of the same acoustic length as the first pipe, being arranged around the first pipe and having a sufliciently larger diameter to form an annular sound path around said first pipe, said first pipe being connected to the second pipe through an annular opening disposed midway between the ends of the second pipe to form two sound radiators having annular mouths located in two parallel planes spaced apart one half of the wave length of the frequency of said oscillatory structure.

4. A directional air sound apparatus, comprising in combination a vibration generator,

an oscillatory structure actuated by said generator and being tuned to a definite wave length, a sound 1 sonator of the organ pipe type having a length of onehalf of the wave length of said frequency and being coupled with said oscillatory structure, a second pipe resonator of the same acoustic len h as the first pipe, being arranged aroun the first pipe and having a sufliciently larger diameter to form an annular sound path around said first pipe, said first pipe being connected to the second pipe through an annular opening disposed midway between the ends of the second pipe to form two sound radiators having annular mouths located in two parallel planes spaced apart one half of the wave length of the frequency of said oscillatory structure, the diameter of said mouths not essentially exceeding onequarter of said wave length.

5. A directional air sound apparatus, comprising in combination a vibration generator an oscillatory structure actuated by said generator and being tuned to a definite wave length, a sound resonator of the organ pipe type having a length of one half of the wave length of said frequency and being coupled with said oscillatory structure, a second pipe resonator of the same acoustic length as the first pipe, being arranged around the first pipe and having a sufliciently larger diameter to form an annular sound path around said first pipe, said first pipe being connected to the second pipe through an annular opening disposed midway between the ends of the second pipe to form two sound radiators having annular mouths located in two parallel planes spaced apart one half of the wave length of the frequency of said oscillatory structure,

.the cross-sections of the sound path in said radiator increasing in size following each break in direction, the ratio of adjacent cross sections including a break increasing with the angle at which the break occurs.

6. A directional air sound apparatus, comprising in combination a vibration generator,

tures.

HEINRICH HECHT. ULRICH JOHN.

an oscillatory structure actuated by said generator and being tuned to a definite wave length, a sound resonator of the organ pipe type having a length of one half of the wave length of said frequency and being coupled with said oscillatory structure, a second pipe resonator of the same acoustic length as the first pipe, being arranged around the first pipe and having a sufliciently larger diameter to form an annular sound path around said first pipe,-said first pipe being connected to the second pipe through an annular opening disposed midway between the ends of the second pipe to form two sound radiators having annular mouths located in two parallel planes spaced apart one half of the wave length of the frequency of said oscillatory structure, the cross-sections of the sound path in said radiator increasing in size following each'break in direction, the ratio of adjacent cross-sections including a break increasing with the angle at which the break occurs and lying between the values 1:1 and 1 :5. I

7 An air sound apparatus comprising in 

